1. Field of the Invention
The present invention relates to an output circuit. More particularly, the present invention relates to a low differential output voltage circuit.
2. Description of Related Art
In recent years, diverse electronic products have been developed. In order to realize communication between electronic products, between integrated circuits, or between various functional modules in integrated circuits to optimize functions of each product, various transmission interfaces came into being.
In order to reduce electromagnetic interference (EMI) and power consumption, transmission interfaces are mostly designed to be a differential output type, for example, a differential output circuit in FIG. 1. FIG. 1 is a schematic view of a conventional differential output circuit and an arrangement thereof. Referring to FIG. 1, the differential output circuit is disposed in a chip, and the circuit includes a differential output unit composed of metal-oxide-semiconductor (MOS) transistors 102-112 and resistors 114, 116, and a feedback circuit 130. The differential output unit outputs a differential signal, and work together with the feedback circuit 130 to perform a feedback control of the common mode voltage Vcm.
In FIG. 1, Vout+ and Vout− are output signals of the circuit, for transmitting communication messages to a receiving end (e.g. another chip, not shown) outside the chip, and Vin+ and Vin− are input signals of the circuit. Moreover, VCC is the source voltage, GND is the grounding voltage, and Vp is the bias provided to the transistor. Transistors 104 and 106 form an input differential pair, transistors 108 and 110 form another input differential pair, and transistors 102 and 112 serve as the current source of the input differential pairs. Resistors 114 and 116 are used to connect the output signals Vout+ and Vout− in series to obtain a common mode voltage Vcm of the two output signals. Resistors 118 and 120 are terminal resistors of the circuit and a receiving end for impedance matching to reduce the interference caused by signal reflection.
After acquiring the common mode voltage Vcm, the feedback circuit 130 will compare the common mode voltage Vcm with a predetermined reference voltage, for example, 1.25V to output a control signal Vn to a gate of the transistor 112 and further control the current of the transistor 112, so that the common mode voltage Vcm can be kept at 1.25V. However, it is necessary for the differential output unit to perform the feedback control through the feedback circuit 130, so the response time of the transmission interface of this type is limited, and the optimized conditions will drift with the variation of the processes.
In addition, as a common chip usually has a plurality of differential output units, and each of the differential output units works needs one feedback circuit to perform the feedback control, the cost is greatly increased. It can be seen from the above disadvantages that the more the differential signals to be transmitted by the system are, the more complex and larger the circuit scale is.